1. Field of the Invention
The present invention relates to a photomask for focus monitoring, a method of focus monitoring, a unit for focus monitoring and a manufacturing method of the electronic device.
2. Description of the Background Art
Increases in the integration and the miniaturization in semiconductor integrated circuits have been remarkable in recent years. Together with that, the miniaturization of the circuit pattern formed on a semiconductor substrate (hereinafter referred to simply as a wafer) has greatly progressed.
In particular, photolithographic technology is widely recognized as a basic technology in the pattern formation. Accordingly, a variety of developments and improvements have been carried out up to the present time. However, the miniaturization of patterns shows no signs of slowing down and demand for increase in resolution of the patterns is on the increase.
Such a photolithographic technology is a technology for transcribing patterns from a photomask (original image) to a photoresist applied on a wafer so that an etched film in the lower layer is patterned by using this transcribed photoresist.
At this time of photoresist transcription, a development treatment is carried out on the photoresist and a photoresist wherein the portion hit by light through this development treatment is removed is called a positive type while a photoresist wherein the portion hit by light is not removed is called a negative type photoresist.
In general, the resolution limit R (nm) in photolithographic technology using a downscaling exposure method is represented as:R=k1·λ/(NA) Here, λ is the wavelength (nm) of the utilized light, NA is the numerical aperture in the projection optical lens system and k1 is a constant depending on the image formation condition and the resist process.
As is seen from the above equation, there is a method of making the values of k1 and λsmaller and of making the value of NA larger in order to achieve an increase in the resolution limit R, that is to say, to gain microscopic patterns. That is to say, in addition to making the constant, which depends on the resist process, smaller, a shortening of the wavelength and an increase of NA may be implemented.
From among these, a shortening of the wavelength of the light source is technically difficult and, therefore, it becomes necessary to increase the NA for the same wavelength. When an increase in NA is implemented, however, the focal depth δ(δ=k2·λ/(NA)2) of light becomes shallow and, therefore, there are problems such that deterioration in form and in dimension precision of formed patterns is caused.
In order to expose a photoresist according to the patterns of a photomask with a high resolution using such photolithographic technology, it is necessary to carry out the exposure under the condition wherein the photoresist accords with the optimal image formation surface (optimal focus surface) of the projection optical system within the range of the focal depth. Therefore, it is necessary to precisely find the distance from the surface of the exposed substrate to the projection optical system. The process of finding this distance is called focus monitoring.
Concerning conventional focus monitoring, there is, for example, the method of phase shift focus monitoring developed by Brunner of IBM Corporation and sold by Benchmark Technology Corporation of the United States and the phase shift focus monitoring mask that is used in this method.
FIG. 56 is a view for describing the operational principle of the method of phase shift focus monitoring. In reference to FIG. 56, a phase shift focus monitoring mask 105 is used in this method of phase shift focus monitoring. This phase shift focus monitoring mask 105 has a transparent substrate 105a, a light blocking film 105b having a predetermined pattern and a phase shifter 105c that is formed on this predetermined pattern.
Concretely, this phase shift focus monitoring mask 105 has a pattern wherein a thin light blocking pattern 105b is arranged between sufficiently thick transmission portions 105d and 105e, as shown in FIG. 57. Here, a phase shifter 105c is not placed in transmission portion 105d while a phase shifter 105c is placed on transmission portion 105e. 
In reference to FIG. 56, in this method of phase shift focus monitoring, first, phase shift focus monitoring mask 105 is irradiated with light. At this time, since phase shifter 105c is formed so as to shift the phase of the transmission light by approximately 90°, in the case that the light that has passed through transmission portion 105e precedes the light that has passed through the transmission portion 105d by the optical path difference of 1/4 λ, 5/4 λ . . . , or in the case that the light that has passed through transmission portion 105e succeeds the light that has passed through the transmission portion 105d by the optical path difference of 3/4 λ, 7/4 λ . . . , the light acts in a mutually reinforcing manner. Thereby, the light after passing through phase shift focus monitoring mask 105 has an asymmetric intensity distribution with respect to the z axis (optical axis). This light that has passed through phase shift focus monitoring mask 105 is condensed by means of projection lens 119a and 119b so as to form an image on a photoresist 121b, which is on a semiconductor substrate 121a. 
According to this method of phase shift focus monitoring, an image is formed on photoresist 121b under the condition wherein the intensity distribution of the diffracted light is asymmetric with respect to the z axis (optical axis: the longitudinal direction in the figure). Therefore, an image of the pattern shifts in the direction (x-y direction: lateral direction in the figure) perpendicular to the z axis (optical axis) on wafer 121 due to the shift of wafer 121 in the z direction. By measuring this amount of shift of the image of the pattern in the x-y direction, the measurement of the position in the z direction, that is to say the measurement of the focus, becomes possible.
In addition to the above described method of phase shift focus monitoring there is a method disclosed in, for example, Japanese Patent Laying-Open No. 6-120116(1994) that is a method of focus monitoring. In this method, a first predetermined pattern in the photomask surface is first irradiated with an exposure light of which the main light beam has the first angle of inclination and, thereby, the first image of the first predetermined pattern is exposed on a substrate of photosensitive material. After that, a second predetermined pattern that is different from the above first predetermined pattern is irradiated with an exposure light of which the main light beam has a second angle of inclination that differs from the first angle of inclination and, thereby, the second image of the second predetermined pattern is exposed on the substrate of the photosensitive material. By measuring the distance between the exposed first and second images, the distance from the position of the substrate of the photosensitive material to the optimal image formation surface can be found from the relationship between this distance and the amount of defocus.
In this method, a predetermined pattern on the photomask surface is irradiated according to the first angle of inclination or according to the second angle of inclination and, therefore, a photomask 205 having the structure as shown in FIG. 58 is used.
In reference to FIG. 58, this photomask 205 has a transparent substrate 205a, marks for position measurement 205b1 and 205b2 formed on the surface of this transparent substrate 205a and a diffraction grid pattern 205c formed on the rear surface of transparent substrate 205a. That is to say, an exposure light that has struck photomask 205 is diffracted by diffraction grid pattern 205c so that mark for position measurement 205b1 is irradiated according to the first angle of inclination and mark for position measurement 205b2 is irradiated according to the second angle of inclination.
In the above described phase shift focusing monitor, however, it is necessary to use a phase shift mask of a specific structure as photomask 105. There is a problem point that the photomask becomes expensive because a photomask of such a specific structure is necessary.
In addition, in a conventional method of phase shift focus monitoring, it is necessary to use illumination which is isotropic (pupil plane is circular) and of which the angle spread is small, that is to say that has a small a value, in order to gain a high detection sensitivity in the z direction (ratio of the of shift amount in the x-y direction to the shift amount in the z direction). This is described in T. A Brunner et al., “Simulations and experiments with the phase shift focus monitor,” SPIE Vol. 2726, pp. 236-243. In particular, FIG. 4 of the above reference shows that when the σ value is 0.3, the shift amount in the x-y direction of the pattern (focus monitor overlay error) becomes of the maximum and the detection sensitivity in the z direction becomes high.
It is necessary to reduce the diameter of aperture 14d of illumination aperture unit 14, such as the illumination diaphram as shown in, for example, FIG. 59.
However, when the device pattern is formed by using illumination of which the σ value is small, such as approximately 0.3, the coherence of the light is too intense and transformation of the secondary pattern transcribed to the photoresist becomes significant. In order to prevent such transformation of the secondary pattern, it is necessary to make the σ value be, for example, 0.6 or higher, by making the diameter of the aperture of the illumination aperture unit 14 used at the time device pattern formation greater than the diameter of the aperture of the illumination aperture unit 14 used at the time of focus monitoring. Therefore, illumination aperture unit 14 must be replaced at the time between focus monitoring and device pattern formation and, therefore, there is a problem that labor and maintenance become necessary for the replacement.
In addition, since at the time of replacement the lenses become clouded in the case that an oxygen mixture remains in the illumination optical system, it is necessary to carry out oxygen purging by introducing nitrogen for a long period of time after the replacement and, therefore, there is a problem such that the operation becomes complicated.
In addition, in a method disclosed in the Japanese Patent Laying-Open No. 6-120116(1994), it is necessary to form a diffraction grid pattern 205c on the rear surface of a photomask 205 as shown in FIG. 58. It is necessary for this diffraction grid pattern 205c to be a microscopic pattern so as to allow light to diffract. There is a problem point that the process dimensions become small because of the necessity for forming such a microscopic pattern and the fabrication of the photomask becomes difficult.
In addition, it is necessary to illuminate only the portion of the rear surface of photomask 205 where diffraction grid pattern 205c exists with an exposure light and, therefore, there is also a problem that the illumination range must be concentrated in one small portion.